1. Field of the Invention
The present invention relates, in general, to an ink jet recording apparatus having sheet or liquid path for containing liquid as an ink container, and provides an advantageous technique in particular for an ink jet recording apparatus equipped with a recording head of bubble jet type which generates bubbles by rapid heating with the electro-thermal converting element thereby emitting ink droplets.
2. Related Background Art
A recording head of the bubble jet type, utilizing bubbles generated by thermal energy as the ink droplet forming means, is capable of significantly reducing the area of activating element, thereby realizing a high-density and compact recording head and eventually compactizing the recording apparatus.
Also the fluctuation in the function among different heads can be reduced since the electro-thermal converting elements can be precisely formed with a photolithographic process.
Next, U.S. Pat. Nos. 4,376,945 and 4,719,472 disclosing a technique for desirably recording with liquid temperature maintained constant are known as a preliminary heating technique of the ink jet recording apparatus. Further, as a technique for modifying the preliminary heating according to an environmental condition or sequence for conducting preliminary heating necessarily on turning on a main switch, British patent 2159465B and British publication 2169855A disclose a speed heating by increasing preliminary heating pulse width without liquid emission.
Among those preliminary heating systems, one disclosed in U.S. Pat. No. 4,463,359 is to add pulse for preliminary heating to the drive pulse for recording and is a very important and advantageous invention as a technique for achieving stabilized recording.
While, as a technique for preliminary emission independent of actual recording, there are British patent 2159465B and British patent publication 2169855A that disclose more concrete examples of an ink jet technique to stabilize recording in accordance with an environmental condition.
As described in the above, in the field of the ink jet recording, excellent preliminary emission means is known and used as shown in the above documents.
Problems not noticed in the above documents have been found after extensive research. The background of the present invention is described as follows. That is, an ink jet recording apparatus, particularly the recording head of serial type, often provides, in the recorded image, a record area of lower density in comparison with other areas.
FIG. 6 shows a sample of recording with a conventional bubble jet recording apparatus, for explaining the above-mentioned phenomenon, wherein 1A-1D are recording regions, including regions 2 of lower density. There is employed a recording head of serial type, which effects recording by scanning motions in a direction indicated by an arrow. As will be apparent from this figure, the lower density appears at the start of recording of each line, and at the start of recording after an unrecorded region.
The cause of this phenomenon will be explained in the following, with reference to FIGS. 4 and 5.
FIG. 4 shows the change in the diameter of recording dots, as the function of lapse of time in each of the recording regions 1A-1D shown in FIG. 6. It will be apparent, from this figure, that the diameter of recording dots varies considerably between the start and end of the recording in a scanning line.
A cause of this phenomenon is that, in a recording head of bubble jet type, a large electric power has to be applied instantaneously to a electro-thermal converting element for generating the bubble, and the heat dispersion is not conducted sufficiently because of the high-speed drive, so that the electro-thermal converting element and the ink are heated with the lapse of recording time, thus causing a change in the rate of expansion or contraction of bubbles and in the viscosity of ink.
However, in the actual recording, the change in the diameter of recorded dots is not directly reflected in the change in the density of recorded image. More specifically, if the ratio of the actual area of dots to the area of pixel in which said dots are placed (said ratio being hereinafter referred to as the area factor) is low, the density of the pixel is influenced by the low density, for example white, of the background of the recording sheet, and the record density is lowered. Thus, when the area factor is relatively small as shown in FIGS. 5(a) to (c), the change in the diameter of recording dots significantly affects the density of the recorded image. On the other hand, when the diameter of recording dots is increased to an area factor of about 100% as shown in FIGS. 5(d) and (e), the density of the recorded image is not too much affected by a slight change of the diameter of recording dots.
Consequently the use of a larger dot diameter is preferable for reducing the change in the image density, but an excessively large dot diameter is not desirable becuase of requires an excessively long time for the ink to be fixed on the sheet. Thus, if the dot diameter is so selected as to ensure rapid fixing even at the end of a scanning line where the dot diameter increases, the area factor decreases in the pixels in the initial portion of the recording, thus resulting a decrease in the recorded image density.